The invention relates to a method for smoothening a web in a papermaking operation before it is coated. The steps in the smoothening operation include calendering a moistened web in a heated hard nip. This process improves the receptivity of the web surface to coating applications, and to subsequent printing operations.
It is known that an otherwise untreated web of paper or paperboard rawstock will react adversely to the application of a liquid by becoming rougher, since the surface fibers of the web absorb moisture leading to fiber swelling, breaking of hydrogen bonds and fiber reorientation. If the liquid application is in the form of a coating or the like, the web may actually become smoother because of the presence of smoothing ingredients in the coating, but the improvement in smoothness is not as great as would be expected because the surface of the rawstock beneath the coating becomes roughened in the presence of the liquid phase of the coating. Because of this phenomenon, and because one of the objectives in the production of paper and paperboard is to produce a smooth finish, it is desirable to smoothen the surface of the rawstock web before coating.
One of the known techniques for smoothing the surface of a web of paper or paperboard rawstock before coating is to pre-calender the web. This may be accomplished on a paper machine after the web is dried. In the pre-calendering step, the web is passed through one or more nips between the rolls of a calender, the nips providing a degree of compression to the paper surface to flatten the surface fibers, thereby creating a smoothened surface. The degree of finish obtained by this method depends upon the nip pressure reached in the calender. However, the smoothness obtained by pre-calendering a dried web is short lived when the web is subsequently brought into contact with water or a composition containing water, since dry cellulose fibers will not bond under pressure. On wetting, there will be substantial reorientation of the fibers at the web surface due to fiber swelling and release of stress. An improvement that alleviates this unwanted result is to moisten the dry web before pre-calendering. The compressing of moistened cellulose fibers will consolidate the web without undue stress, and the smoothened surface that is achieved will retain a substantial amount of its integrity for a longer time after wetting than is common for a web that is compressed in an essentially dry state.
Conventional processes for wet calendering employ either a water box on the calender, water sprays, or steam showers. In most cases, the quantity of moisture transferred to the web by these means is uneven, too great or too small, resulting in unsatisfactory results. Nevertheless, recent innovations in the paper industry have provided equipment that is capable of applying controlled amounts of water to a web in a uniform manner, particularly as disclosed in pending U.S. patent application Ser. No. 09/296,394, owned by the present assignee herein. The ability to apply a controlled amount of moisture to the web at the pre-calender yields better results. Nevertheless, there remains a need in the art to further enhance the process, and, accordingly, the present invention is designed to advance the process efficiency and product quality that can be achieved in a pre-calendering operation.
The importance of having a smooth rawstock surface and its effect on the printability of coated paper is well known. Pigmented coatings tend to hide or cover up some of the surface imperfections in a paper rawstock. However, the effectiveness of a coating in smoothing the surface of a paper web depends upon the condition of the surface on which the coating is applied. The effect on paper due to the application of a coating is discussed in the article entitled xe2x80x9cWater-Paper Interaction During Paper Coatingxe2x80x9d, by J. Skowronski and P. Lepoutre, published in the November 1985 issue of TAPPI Journal, at pages 98-102. Meanwhile, the effect of pretreatment or pre-calendering of a paper product before coating is discussed in the articles xe2x80x9cEffect of Pretreatment of LWC Basestock on Coated Paper Propertiesxe2x80x9d, by P. Lepoutre, W. Bichard and J. Skowronski, published in the December 1986 issue of TAPPI Journal, at pages 66-70, and in the article entitled xe2x80x9cPrecalendering and its Effect on Paper-Coating Interactionxe2x80x9d, by G. Engstrom and J. F. Lafaye, published in the August 1992 issue of TAPPI Journal, at pages 117-122.
Likewise, the effect of applying moisture to a paper web before the web enters a calender nip, and various methods for making water application are also known, and are shown, for example, in U.S. Pat. Nos. 5,378,497; 5,505,820; 5,522,312; and 5,607,553, all owned by the present assignee herein. However, none of these prior art references deal with the concept of smoothening the web while reducing loss of bulk by drying the web to a very low moisture content before the application of moisture, passing it through a hard calender nip, and heating at least one roll of the calender nip as disclosed herein. Nor do the prior art references teach the use of the heated roll to dry the applied moisture as the web exits the calender nip as disclosed herein, especially in connection with a pre-calendering treatment on a paper machine.
The apparatus comprises at least one heated nip formed between two rolls, at least one of which is heated. The heated nip may be formed between one heated and one unheated roll or it may be formed between two heated rolls in a calender stack. The moisture application is made to the surface of the web that contacts the heated roll. One nip with controlled loading force will normally suffice, but multiple nips, in tandem or stacked may preferably be employed.